I. Field of the Invention
This invention relates generally to an apparatus and method for treating and assessing the efficacy of such treatment of CHF patients, and more particularly to an improved cardiac rhythm management device (pace only or pace/defibrillator combination) incorporating circuitry for sensing and storing respiratory and activity-related parameters and for telemetrically transmitting such parameters to an external monitor, upon demand, for analysis by medical professional in an out-patient setting.
II. Discussion of the Prior Art
"Cardiac Pacemakers" and "Cardiac Rhythm Management" (CRM) devices are both used as generic terms for pacemakers and defibrillators. CRMs have long been used in the treatment of bradycardia and tachycardia, but only more recently have been specifically designed to enhance the hemodynamic performance of the heart as a pump for patients suffering from CHF. Pacemakers designed for treatment of bradycardia have incorporated a variety of physiologic sensors whereby the pacing rate of the pacemaker can be made to vary automatically with changes in physiologic demand. Thus, for example, the Hauck et al. U.S. Pat. No. 5,318,597 describes a rate adaptive pacemaker in which the pacing rate is adjusted in accordance with changes in a patient's minute ventilation. Means are provided in the CRM for measuring variations in transthoracic impedance and for signal processing the impedance signal to extract a minute ventilation signal that is used to vary the pacing rate of an implantable pacemaker between lower and upper programmed rate limit values in that there is a direct correlation between a patient's minute ventilation and the body's hemodynamic demand.
Other rate adaptive CRMs have incorporated some form of an activity sensor, such as an accelerometer, for developing a control signal that varies with a patient's level of physical activity. This control signal is then used to vary the pacing rate of a rate adaptive CRM so as to maintain an appropriate pacing rate for the level of exercise being exerted.
In implantable CRM devices especially designed for treating patients with CHF, one approach that has proved successful involves the automatic optimization of the AV delay of an implantable, dual-chamber pacemaker. For a general description of the prior art relating to pacemakers for treating CHF, reference is made to the Baumann Patent U.S. Pat. No. 5,800,471, the teachings of which are incorporated herein by reference as if set forth in full.
The present invention constitutes a departure from the prior art in that even though the apparatus employed incorporates circuitry for implementing impedance plethysmography in generating a signal component relating to minute ventilation and an accelerometer responsive to patient activity, the information derived from these sensors is used to monitor a CHF patient's status so that the efficacy of a change in pacing therapy or drug therapy can be read from the implanted device periodically for review and analysis by a medical professional.
Studies have shown that patients with chronic heart failure are limited by exertional dyspnea and exercise intolerance. Such patients often exhibit elevated ventilatory response to exercise, which can be characterized by a steeper slope relating minute ventilation to carbon dioxide output during exercise. In addition to the increased ventilation, such patients have also been noted to have an abnormal breathing pattern, such that at a given minute ventilation, respiratory rate is increased while the change in tidal volume is less significant compared with normal subjects. The ventilatory response to exercise, as characterized by the regression slope relating minute ventilation to carbon dioxide output during exercise by CHF patients, has been found to be significantly higher in such patients than for normal subjects.
See "The Role of peripheral Chemoreflex in Chronic Congestive Heart Failure" by T. P. Chua et al, CHF; November/December 1997; pp. 22-28.
The method of the present invention is carried out by implanting in the patient a CRM device of a type incorporating a transthoracic impedance sensor, a patient activity sensor and a microprocessor operatively coupled to the transthoracic impedance sensor for controlling the delivery of cardiac stimulating pulses to the patient's heart. The microprocessor is programmed to permit it to derive a plurality of respiratory parameters from signals produced by the transthoracic impedance sensor. A telemetry link allows the respiratory parameters or signals relating thereto to be transmitted to an external monitor for viewing by a medical professional. Either the microprocessor forming a part of the implanted CRM device or a microprocessor contained within the external monitor computes predetermined ratio values involving the derived respiratory parameters as congestive heart failure therapy status indicators.
The apparatus involved may further include an activity sensor connected to the microprocessor for deriving a plurality of physiologic parameters based upon the signal output of the activity sensor. The activity sensor derived parameters may also be telemetered to the external monitor for display or for further processing and display.
Meaningful ratios that serve as CHF status indicators include ventilatory tidal volume to respiratory rate tv/RR, ventilatory tidal volume to inspiratory time tv/it and minute ventilation to carbon dioxide output MV/VCO.sub.2. Another ratio of interest in assessing CHF status is O.sub.2 pulse, which is the amount of oxygen uptake per heartbeat VO.sub.2 /HR.
Other researchers have found a high correlation between oxygen consumption (VO.sub.2) and carbon dioxide production (VCO.sub.2) with heart rate and physical activity to the point where it is possible to estimate these respiratory parameters based on measured heart rate and physical activity. See, for example, "Combined Heart Rate and Activity Improve Estimates of Oxygen Consumption in Carbon Dioxide Production Rate" by Jon K. Moon and Nancy F. Butte, J. Applied Physiology, 81(4:1754-1761) 1996. The present invention takes advantage of such high correlation, providing a way in which a particular pacing or drug therapy affects the status of CMF patients in whom the present invention is utilized.